\documentclass{article}

\usepackage[a3paper, landscape]{geometry}
\usepackage{graphicx}
\usepackage{pdfpages}

\pagestyle{empty}

\begin{document}

\includepdfmerge[nup=2x1, noautoscale, delta=14 0, pages={last-last, 1}]{spine.pdf, ../thesis.pdf, 1}
%\includepdfmerge[nup=2x1, noautoscale, offset=\pageoffset{} 0,
%delta=\pagedelta{} 0, pages={last-last,1}]{spine.pdf, ../main.pdf, 1 }

\end{document}
